Systems and methods for producing white-light light emitting diodes

ABSTRACT

A vertical light emitting diode (LED) includes a metal substrate; a p-electrode coupled to the metal substrate; a p-contact coupled to the p-electrode; a p-GaN portion coupled to the p electrode; an active region coupled to the p-GaN portion; an n-GaN portion coupled to the active region; and a phosphor layer coupled to the n-GaN.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/032,853, filed Jan. 11, 2005 now U.S. Pat. No. 7,195,44, and isherein incorporated by reference.

The present invention generally relates to a light-emitting diode and amethod for fabricating the same.

Advances in light emitting diode (LED) technology have resulted in LEDswith characteristics of small volume, light weight, high efficiency, andlong life. These LEDs have seen great advances in differentmonochromatic color output, such as red, blue, and green. Single colorLED's can be used as a backlight in a special display, for instance,mobile phones and light crystal displays (LCDs).

Recently, various attempts have been made to make white light sources byusing light emitting diodes. Because the light emitting diode has afavorable emission spectrum to generate monochromatic light, making alight source for white light requires it to arrange three light emittingcomponents of R, G and B closely to each other while diffusing andmixing the light emitted by them. When generating white light with suchan arrangement, there has been such a problem that white light of thedesired tone cannot be generated due to variations in the tone,luminance and other factors of the light emitting component. Also whenthe light emitting components are made of different materials, electricpower required for driving differs from one light emitting diode toanother, making it necessary to apply different voltages different lightemitting components, which leads to complex drive circuit. Moreover,because the light emitting components are semiconductor light emittingcomponents, color tone is subject to variation due to the difference intemperature characteristics, chronological changes and operatingenvironment, or unevenness in color may be caused due to failure inuniformly mixing the light emitted by the light emitting components.Thus light emitting diodes are effective as light emitting devices forgenerating individual colors, although a satisfactory light sourcecapable of emitting white light by using light emitting components hasnot been obtained so far.

U.S. Pat. No. 5,998,925 discloses a white light emitting diode having alight emitting component that uses a semiconductor as a light emittinglayer and a phosphor which absorbs a part of light emitted by the lightemitting component and emits light of wavelength different from that ofthe absorbed light, wherein the light emitting layer of the lightemitting component is a nitride compound semiconductor and the phosphorcontains garnet fluorescent material activated with cerium whichcontains at least one element selected from the group consisting of Y,Lu, Sc, La, Gd and Sm, and at least one element selected from the groupconsisting of Al, Ga and In and, is subject to less deterioration ofemission characteristic even when used with high luminance for a longperiod of time.

FIG. 1 shows a lead type LED as disclosed in the '925 patent. The lightemitting diode is a lead type light emitting diode having a mount lead 2and an inner lead 4, wherein a light emitting component 8 is installedon a cup 6 of the mount lead 2, and the cup 6 is filled with a coatingresin 14 which contains a specified phosphor to cover the light emittingcomponent 8 and is molded in resin. An n electrode and a p electrode ofthe light emitting component 8 are connected to the mount lead 2 and theinner lead 4, respectively, by means of wires 12. In the light emittingdiode constituted as described above, part of light emitted by the lightemitting component (LED chip) 8 (hereinafter referred to as LED light)excites the phosphor contained in the coating resin 14 to generatefluorescent light having a wavelength different from that of LED light,so that the fluorescent light emitted by the phosphor and LED lightwhich is output without contributing to the excitation of the phosphorare mixed and output. As a result, the light emitting diode also outputslight having a wavelength different from that of LED light emitted bythe light emitting component 8.

FIG. 2 shows a chip embodiment as disclosed in the '925 patent. The chiptype LED 26 is installed in a recess of a casing 22 which is filled witha coating material which contains a specified phosphor to form a coating28. The light emitting component 26 is fixed by using an epoxy resin orthe like which contains Ag, for example, and an n electrode and a pelectrode of the light emitting component 26 are connected to metalterminals 20 installed on the casing 22 by means of conductive wires 24.In the chip type light emitting diode constituted as described above,similarly to the lead type light emitting diode of FIG. 1, fluorescentlight emitted by the phosphor and LED light which is transmitted withoutbeing absorbed by the phosphor are mixed and output, so that the lightemitting diode also outputs light having a wavelength different fromthat of LED light emitted by the light emitting component 26.

U.S. Pat. No. 6,642,652 discloses a light source that includes a lightemitting device such as a III-nitride light emitting diode covered witha luminescent material structure, such as a single layer or multiplelayers of phosphor. Any variations in the thickness of the luminescentmaterial structure are less than or equal to 10% of the averagethickness of the luminescent material structure. In some embodiments,the thickness of the luminescent material structure is less than 10% ofa cross-sectional dimension of the light emitting device. In someembodiments, the luminescent material structure is the only luminescentmaterial through which light emitted from the light emitting devicepasses. In some embodiments, the luminescent material structure isbetween about 15 and about 100 microns thick. The luminescent materialstructure is selectively deposited on the light emitting device by, forexample, stenciling or electrophoretic deposition.

An LED coated with phosphor according to the '652 patent is illustratedin FIG. 3. The LED in FIG. 3 includes a n-type region 44 formed on asubstrate 42, such as sapphire, SiC, or a III-nitride material. Anactive region 46 is formed on the n-type region 44, and a p-type region36 is formed on the active region 46. N-type region 44, active region46, and p-type region 36 are typically multiple-layer structures. Aportion of the p-type region 36, the active region 46, and the n-typeregion 44 is etched away to expose a portion of n-type region 44. Ap-type contact 34 is deposited on the p-type region 36 and an n-typecontact 38 is deposited on the exposed portion of n-type region 44. TheLED is then flipped over and mounted to a sub-mount 30 by a material 32such as solder.

U.S. Pat. No. 6,744,196 discloses thin film LED devices comprised of LEDchips that emit light at a first wavelength, and a tinted thin filmlayer over the LED chip that changes the color of the emitted light. Forexample, a blue-light emitting LED chip can be used to produce whitelight. The tinted thin film layer beneficially consists of ZnSe, CeO₂,Al₂O₃, or Y₂O₃Ce that is deposited using a chemical vapor depositionprocess, such as metal organic chemical vapor deposition (MOCVD), atomiclayer chemical vapor deposition (ALD), plasma enhanced MOCVD, plasmaenhanced ALD, and/or photo enhanced CVD. As shown in FIG. 4, anN-contact 50 is positioned below a reflective layer 52. A tinted layer(a phosphor layer) 53 is positioned above the reflective layer 52. Next,a first passivation layer 54 is formed, and a p semi-transparent contact56 is formed. A second passivation layer 58 is formed above the firstpassivation layer 54 and contact 56. A conductive wire 60 is connectedto a p-pad 62, which is positioned above the p-lead 64.

SUMMARY

A vertical light emitting diode (LED) includes a metal substrate; ap-electrode coupled to the metal substrate; a p-GaN portion coupled tothe p electrode; an active region coupled to the p-GaN portion; an n-GaNportion coupled to the active region; and a phosphor layer coupled tothe n-GaN.

Implementations of the above LED can include one or more of thefollowing. The metal substrate with a mirror layer is formed on top ofthe p-GaN portion; using laser lift-off (LLO), selective wet etching orchemical mechanical polishing, the sapphire carrier is removed. Thep-contact can also be a light reflector. The phosphor layer can bespin-coated or screen printing with a phosphor powder or paste. Thephosphor layer is patterned using a masking material 101 such as photoresist and dry etch using a fluorine containing plasma; then a metallayer Cr/Ni 99 such as Cr/Ni is applied to form a contact with n-GaNusing various techniques such as PVD, ebeam evaporation or CVD; themetal bond pad 98 is formed a contact with n-GaN after the resist 101 isremoved and lift-off the unwanted areas of metal 99 to form bond pad 98.This is called resist lift-off techniques. The phosphor layer and thebond pad cover the exposed n-GaN surface 80.

Advantage of the invention may include one or more of the following. Theabove LED wafer surface with exposed n-GaN portion 80 layer issubstantial smooth, planar surface for subsequent process. The methodlowers the cost of producing white LED by coating the phosphor on theblue LEDs at wafer level directly on top of exposed n-GaN surface ascomparing to conventional one LED die at a time. The method reduces theamount of phosphor needed for each die by cover only the exposed n-GaNsurface. The LEDs do not require wafer bonding or gluing and the complexand lengthy and one at a time wafer bonding/gluing process is replacedby a less complex deposition process for example physical vapordeposition (PVD), chemical vapor deposition (CVD), plasma enhanced CVD(PECVD), evaporation, ion beam deposition, electro chemical deposition,electroless chemical deposition, plasma spray, or ink jet deposition. Nosemi-transparent contact is needed for the n-electrode since n-GaNconductivity is good, and as a result, more light output can be emittedfrom the LED device. Further, since only one electrode is needed on eachside of the device, the LED electrode obstructs less light.Additionally, current can spread out uniformly from n-electrode top-electrode, thus increasing LED performance. Moreover, the metalsubstrate can dissipate more heat than the sapphire substrate, so morecurrent can be used to drive the LED. The resulting LED can replace theconventional LED at a smaller size. For the same LED size, the lightoutput from vertical LED is significantly higher than the conventionalLED for the same drive current.

BRIEF DESCRIPTION OF THE DRAWINGS

To better understand the other features, technical concepts and objectsof the present invention, one may clearly read the description of thefollowing embodiments and the accompanying drawings, in which:

FIGS. 1-4 show various prior art LEDs.

FIG. 5 shows a first embodiment of a vertical LED in accordance with oneembodiment of the invention.

FIG. 6 shows the LED of FIG. 5 with a phosphor coating thereon.

FIG. 7 shows patterned phosphor coating with photo resist masking layer.

FIG. 8 shows metal contact layer deposited on the patterned phosphorcoating of FIG. 7.

FIG. 9 shows patterned metal bond pads and phosphor coating covering theLED wafer.

FIG. 10 shows a plurality of the phosphor coated LEDs.

DESCRIPTION

Next, with reference to the accompanying drawings, embodiments of thepresent invention will be described. In reading the detaileddescription, the accompanying drawings may be referenced at the sametime and considered as part of the detailed description.

FIG. 5 shows an exemplary structure of one embodiment of a vertical-LEDwafer. Each LED includes a metal substrate 70 made from a laser lift-offprocess. A p electrode 72 is positioned above the metal substrate 70.Next, a light reflector and p-contact 74 and a p-GaN portion 76 arepositioned above the p electrode 72. An active region 78 (including amulti-quantum well) is formed, and an n-GaN portion 82 is formed abovethe active region 78; the n-GaN has an exposed surface 80.

The LED is formed by depositing a multilayer epitaxial structure above acarrier substrate such as sapphire; depositing at least one metal layerabove the multilayer epitaxial structure; and removing the carriersubstrate to leave the metal substrate 70. The metal layer can bedeposited using Electro chemical deposition, electroless chemicaldeposition, CVD chemical vapor deposition, MOCVD Metal Organic CVD,PECVD Plasma enhanced CVD, ALD Atomic layer deposition, PVD Physicalvapor deposition, evaporation, or plasma spray, or the combination ofthese techniques. The metal layer can be single or multi-layered. In oneembodiment, Ag/Pt or Ag/Pd or Ag/Cr is used as a mirror layer, Ni isused as a barrier for Gold as a seed layer for copper plating which isused as the bulk substrate. The mirror layer (Ag, Al, Pt, Ti, Cr forexample) is deposited and then a barrier layer such as TiN, TaN, TiWN,TiW with oxygen is formed above the mirror layer before electro orelectroless chemical deposition of a metal such as Ni or Cu. Forelectrochemical deposition of copper, a seed layer is deposited usingCVD, MOCVD, PVD, ALD, or evaporation process; some of the seed materialsfor Copper are W, Au, Cu or Ni, among others. The metal layers couldhave same or different composition and deposited using variousdeposition techniques. The carrier substrate removal can be done usinglaser, etching, grinding/lapping or chemical mechanical polishing or wetetching, among others.

The sapphire substrate can be removed using laser lift-off (LLO)technique. The multilayer epitaxial layer can have a reflective metallayer coupled to the metal plating layer; a passivation layer 81 ispassivating the sidewall of the LED dies, coupled to the reflectivemetal layer 74, p-electrode 72, p-GaN 76, MQW 78, and n-GaN 82; a p-GaNlayer coupled to the passivation layer; a n-GaN layer coupled to the MQWlayer; an n-electrode coupled to the n-GaN layer; and the metal platinglayer is a p-electrode or having a p-electrode coupled to the metalplating layer.

FIG. 6 shows phosphor coating 90 on the vertical-LED. Since the LEDwafer is substantial smooth and planar, the wafer-level phosphor coatingis uniform and parallel to the emitting LED surface 80, colorful ringsdo not exist on the LED's field pattern because the blue light emittedfrom the active layers travels the same distance or light paths throughthe phosphor layer. In addition, the LED thickness (GaN layer totalthickness is about 2 um to 8 um) is much smaller by the emitting surface82 (greater than 50 um). Light is mainly emitted from the top surface,and a few come from the sidewall to minimize issues arising from uneventhickness of the phosphor layer 90.

The phosphor layer 90 can be formed using a spin coater. The phosphorlayer 90 can be coated by the spin-coater spinning between 500 to 30000rpm to control the layer thickness on the n-side-up vertical-LED wafer.In addition to the spin coat method, other methods such as the screenprinting, roller method, or dipping method can be used. In particular,to obtain a predetermined equal film thickness, the spin coat method ispreferably used. After the phosphor is coated on the substrate, thecoated film is dried. The drying method is not limited as long asmoisture of the film is evaporated. Thus, various methods using a heateror dried air or surface treatment such as a radiant heat lamp can beused. Alternatively, the coated film may be dried by leaving it in aroom temperature environment for a long time.

To make the phosphor coating, a phosphor powder composition is prepared.For this purpose, for example, a dispersing agent is dispersed inpurified water, and the dispersion is stirred with a homomixer andplaced in the purified water in which the dispersing agent has beendispersed, and the mixture is stirred. In the phosphor powdercomposition, water can be used as a dispersing medium. The phosphorpowder composition may contain alcohol as a dispersing agent or aretaining agent, and ammonium bi-chromate may be used as aphotosensitive polymer. The phosphor powders may be surface-treated ontheir manufacturing process, for improving the dispersing property andadhesion thereof. The Phosphor coating material is composed by thephosphor elements mixed in organic chemicals such as alcohol, aerosol,binder material or resin epoxy to tune the viscosity of the coatingmaterial. The thickness can be tuned by the material viscosity and spinrate reproducibly to change the resulting CIE coordination of the whitelight LEDs. Next, a photoresist layer 101 is applied and exposed with acontact pattern, then the phosphor layer 90 is etched. The etching caninclude a dry etching method.

FIG. 7 shows patterned phosphor layer with a masking photo resistmasking layer. A patterned phosphor layer 96 is formed on the exposedn-GaN surface 80. The patterned phosphor layer 96 can be patterned usinga dry etching process. In the dry etching process, a photoresistive maskis placed over the phosphor thin films and exposing the thin films to acorrosive gas within an electric field. The mask can includephotoresistive strips corresponding to the dimensions of the phosphorsegments. The result of the etching is a plurality of openings for acontact opening for later depositing a contact metal layer 99 such asNi/Cr (Ni is in contact with n-GaN).

In FIG. 8, a contact metal layer 99 such as Ni/Cr (Ni is in contact withn-GaN) is deposited on top of the photoresist masking layer, couple withphosphor layer 96 and in contact with n-GaN 80. Metal layer 99 can bedeposited using CVD, PVP or ebeam evaporation.

In FIG. 8, a bonding pad 98 is formed above the patterned phosphor layer96 to form an n-pad. The bond pad metal 98 is formed by lift-offtechniques during the removal of the photoresist masking layer 101 usingan aqueous solution such as diluted KOH. The process for phosphorcoating and bonding pad can be exchanged, wherein the n-GaN contactmetal 99 is patterned, dry etched and protected first by a photoresistmasking layer before the phosphor layer 96 is applied and patterned bylift-off technique during the removal of the photoresist masking layerprotecting the bond pad 98 using an aqueous solution, such as dilutedKOH.

FIG. 10 shows a plurality of white LED chips. The phosphor coated LEDwafer is diced into white chips, which can be packaged directly withoutphosphor addition on the chip level. The phosphor coating is integratedinto the n-side-up vertical LED of wafer-level process. Then thewhite-light wafer is diced into separated white-LED chips by laser orsaw.

Although a single phosphor layer is described above, multiple phosphorlayers can be used. For example, a red photosensitive phosphor powdercomposition (phosphor slurry) can be applied to the entire surface,exposed to light and developed, then, a green photosensitive phosphorpowder composition (phosphor slurry) can be applied to the entiresurface, exposed to light and developed, and then a blue photosensitivephosphor powder composition (phosphor slurry) is applied to the entiresurface, exposed to light and developed.

While the invention has been described by way of examples and in termsof preferred embodiments, it is to be understood that the invention isnot limited thereto. On the contrary, it is intended to cover variousmodifications and similar arrangements and procedures, and the scope ofthe appended claims therefore should be accorded the broadestinterpretation so as to encompass all such modifications and similararrangements and procedures.

1. A vertical light-emitting diode (LED) structure, comprising: a metalsubstrate deposited adjacent to a p-electrode; a p-GaN layer disposedabove the p-electrode; an active region for emitting light disposedabove the p-GaN layer; an n-GaN layer disposed above the active region;and a wafer-level phosphor layer and an n-electrode disposed above then-GaN layer, wherein the phosphor layer is parallel to the n-GaN layer.2. The LED structure of claim 1, wherein the metal substrate isdeposited using at least one of electrochemical plating, electrolesschemical plating, sputtering, chemical vapor deposition, e-beamevaporation, and thermal spray.
 3. The LED structure of claim 1, whereinthe metal substrate comprises at least one of copper, nickel, aluminum,and alloys thereof.
 4. The LED structure of claim 1, further comprisinga p-contact interposed between the metal substrate and the p-GaN layer,wherein the p-contact comprises a light reflector.
 5. The LED structureof claim 1, wherein the phosphor layer is spin-coated.
 6. The LEDstructure of claim 5, wherein the phosphor layer comprises a phosphorpowder composition spin-coated on the n-GaN layer.
 7. The LED structureof claim 6, wherein the phosphor powder composition comprises phosphorparticles mixed with one or more organic chemicals.
 8. The LED structureof claim 1, wherein the phosphor layer is patterned.
 9. The LEDstructure of claim 1, wherein the phosphor layer is dry-etched.
 10. TheLED structure of claim 1, wherein the n-electrode is a metal bond padcoupled to the n-GaN layer through the phosphor layer.
 11. The LEDstructure of claim 1, wherein the phosphor layer contacts a surface ofthe n-GaN layer.
 12. The LED structure of claim 11, wherein the surfaceof the n-GaN layer is substantially smooth and planar.
 13. The LEDstructure of claim 1, wherein the thickness of the phosphor layer issubstantially uniform.
 14. A vertical light-emitting diode (LED)structure, comprising: a metal substrate deposited adjacent to ap-electrode; a p-GaN layer disposed above the p-electrode; an activeregion for emitting light disposed above the p-GaN layer; an n-GaN layerdisposed above the active region; a first wafer-level phosphor layer andan n-electrode disposed above the n-GaN layer, wherein the firstphosphor layer is parallel to the n-GaN layer; and a second phosphorlayer coupled to the first phosphor layer.
 15. The LED structure ofclaim 14, wherein at least one of the first and the second phosphorlayers is patterned.
 16. The LED structure of claim 14, wherein thesurface of the n-GaN layer is substantially smooth and planar.
 17. TheLED structure of claim 14, wherein the thickness of the first phosphorlayer is substantially uniform.
 18. The LED structure of claim 17,wherein the thickness of the second phosphor layer is substantiallyuniform.
 19. LED structure of claim 14, wherein the metal substrate isdeposited using at least one of electrochemical plating, electrolesschemical plating, sputtering, chemical vapor deposition, e-beamevaporation, and thermal spray.
 20. A vertical light-emitting diode(LED) structure, comprising: a metal substrate deposited adjacent to ap-electrode; a p-GaN layer disposed above the p-electrode; an activeregion for emitting light disposed above the p-GaN layer; an n-GaN layerdisposed above the active region; a first wafer-level phosphor layer andan n-electrode disposed above the n-GaN layer, wherein the firstphosphor layer is parallel to the n-GaN layer; and a second phosphorlayer coupled to the first phosphor layer, wherein the first phosphorlayer has a phosphor composition that is photosensitive to a differentcolor than a phosphor composition of the second phosphor layer.